The present invention relates to a fluorescent lamp and a process for fabricating the same.
At present, incandescent and fluorescent lamps are widely used for indoor lighting. Although the incandescent lamps have a low efficiency of the order of about 15 lm/W (in the case of 100 W) which is considerably lower than the efficiency of the fluorescent lamps, they are as much used as the fluorescent lamps. The reason is that since the incandescent lamps are simple to operate and compact in size yet capable of highly luminous light, they can readily provide the so-called accentuation lighting. Therefore, they are widely used in homes and shops. However, from the standpoint of energy saving, apparently it is not preferable to use the incandescent lamps with low efficiencies. As a result, there has been a strong demand for electric discharge lamps which have all the advantages of the incandescent lamps and further have a high efficiency.
In an attempt to meet such demand, the same inventors proposed a fluorescent lamp as disclosed in U.S. Pat. No. 4,095,135 (British Pat. No. 1,578,246). This fluorescent lamp not only exhibits a high efficiency but also has all the merits of the incandescent lamps such as compactness. Briefly stated, the fluorescent lamp has generally a spherical or dome-shaped configuration and comprises a spherical or dome-shaped outer bulb and an inner bulb formed with a zig-zag groove over the outer wall surface thereof and inserted into the outer bulb in predetermined nested relationship so as to define a zig-zag discharge path therebetween. The discharge path has a substantially circular or elliptical cross sectional configuration and is filled with a uv radiation-emitting discharge gas consisting of mercury vapor and a rare gas or a rare gas mixture for producing ultraviolet radiation. A phosphor is applied to the inner wall surface of the outer bulb and/or the outer wall surface of the inner bulb.
However, the inventors found out that the step for sealing the open ends of the outer and inner bulbs together had some problems, adversely affecting the productivity of the production line. That is, as compared with the conventional tubular fluorescent lamps, the open ends of the outer and inner bulbs of the fluorescent lamp of the type described are very large in diameter. In addition, when these open ends are sealed, the lead-wires of the electrodes must be also sealed. The inventors conducted many tests for sealing the open ends together by the conventional sealing techniques used in the production of the conventional fluorescent lamps; that is, by melting the open ends and joining them together gas-tightly. The results are that cracks tend to propagate at the sealed portions and that leakages also tend to occur very often at the sealed portions and along the lead-wires connected to the electrodes disposed at the ends of the zig-zag discharge path.